Monohybrid Problems

1. A true- breeding plant that produces yellow seeds is crossed with a true-breeding plant that produces green seeds. The seeds of all the offspring are yellow. Why?

     The answer is the yellow allele is dominant to the green allele. Since the yellow allele is dominant all the offspring will be yellow as well.

2. A true-breeding plant that produces yellow seeds is crossed with a true-breeding plant that produces green seeds The F1 plants have yellow seeds. What is the expected phenotypic ratio of seed color of the offspring of an F1xF1 cross?

     F1 means first generation offspring so the F1 offspring would be heterozygous Aa. So we would cross AaxAa. The offspring would be AA, Aa, Aa, and aa. The question is asking for phenotypic ratio which means the physical traits ratio. Since three of the offspring have a capital A which stands for yellow seed and only one has aa, green seed. the ratio would be 3 yellow seed to 1 green seed.

3. Attached earlobes are recessive to free earlobes. What is the probability of having a child with attached earlobes when an individual with attached earlobes mates with an individual heterozygous for free earlobes?

   In this problem you would cross a recessive individual (aa) and heterozygous individual (Aa). The offspring would be Aa, Aa, aa, and aa. Since half of the offspring are aa (attached earlobes) so the answer would be 50%.

4. To determine the phenotype of an individual who expresses a dominant trait, you would cross that individual with an individual who_____.

   The answer would be homozygous recessive for that trait.

5. In garden peas, tallness is dominant and dwarfness is recessive. A heterozygous tall plant is crossed with a dwarf plant. If 40 offspring are produced, the number of tall individuals will be approximately?

    You would cross a heterozygous individual (Aa) and a recessive individual (aa). The offspring would be Aa, Aa, aa, and aa. Since half of the offspring are aa  and there are 40 offspring. The answer would be 20.

Here is a picture that explains monohybrid crosses and also a little of dihybrid crosses (2 traits)